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74                        ANALYTICAL MI-X'IIAXK'S
consider the velocity with which he is going to land. In this case, therefore, the surface of the earth should he taken as the reference system.
68.  Fundamental Magnitudes.     The first  two of the four conceptions into which we analyzed motion arc similar; therefore three distinct  conceptions  are  associated   with motion.   The first of these is (he idea of body, or of mat ter; the second is that of distance, and (lie third is thai of time. Distance and time are terms which are too familiar to he made clearer by definitions, therefore we will not attempt to define them.
In their efforts to reduce natural phenomena to their simplest terms scientists have come to the conclusion that-all physical phenomena are the result of motion. It is the main object of science to describe tin* complicated phenomena of nature in terms of motion, in other words, to express all physical magnitudes in terms of the three magnitudes involved in motion. Therefore time, mass, and length are called fundamental magnitudes and all others dcriml, magnitudes.
69.   Fundamental Units.—-The units of time, length, and mass are called fundamental unitx, while those of other magnitudes are called derived with.
70.   The Unit of Time is nu.Uo P^rf of the mean solar day, and is called the second.
71.   The Unit of Length is the ccntinu'ter, which is tln} part of the standard meter.   The latter is the distance at ()'''('„ between two parallel lines drawn upon a certain plafimtm-iridium bar in the possession of the French government-.
72.   Mass.—The choice of the units of time and length is comparatively easy.   We associate only one property with •each of these quantities, therefore in choosing a unit all we have to do is to decide upon its me,    Matter, on the other hand, has a great number of properties, such as volume, .shape, temperature, weight, mass, elasticity, etc.    We com-